1. Field of the Invention
The present invention relates to a semiconductor memory device employing bipolar transistors.
2. Description of the Related Art
Bipolar transistors have been used as a current amplifier which receives a base current and outputs a collector current. For instance, when an NPN bipolar transistor is supplied with a positive collector-emitter voltage V.sub.CE and a positive base-emitter voltage V.sub.BE (V.sub.CE &gt;V.sub.BE), the collector current I.sub.C takes an amplified positive value for various values of the base-emitter voltage V.sub.BE, and the base current also takes a positive value. As conventional bipolar transistors function alike, they have limited fields of application.
The present inventor has proposed a semiconductor device employing novel bipolar transistors which can permit a positive base current and a negative base current in accordance with the base potential, as disclosed in U.S. Pat. Application Ser. No. 293,807 (filed Jan. 5, 1989). In this semiconductor device, given that the forward base current between the base and emitter is I.sub.BE and the reverse base current between the collector and base is I.sub.CE, the collector-emitter voltage V.sub.CE is set as to satisfy I.sub.BE &lt;I.sub.CB according to the base potential. In this case, the collector voltage is always kept constant.
According to the mentioned prior art semiconductor device, in a memory cell constituted by a bipolar transistor, it is necessary to set the collector potential high and thus enhance the current drivability of the base in order to charge a capacitor on a bit line at the time of reading data of a high-potential level and prevent data from being influenced by noise generated at the rising or falling of a word line. Since the collector potential is high even while the memory cell is holding data, however, a through current flows to the emitter from the collector, thus increasing current consumption.